Which Type of Reagent Grade Water do I Need?

Common sense tells us that dilution water must contain significantly lower levels of impurities than the sample to be analyzed. Most modern analytical instruments and procedures generally call for Type I water, since they are analyzing at the “parts per billion” or very low parts per million level. Type I water is almost always produced by a “DI polishing system” at the “point-of-use”. Thus, Type I water is not produced and stored for later use, but is produced and used “on the spot” as required. In fact, those who specify reagent grade water quality, specifically state that Type I water should not be stored for later use.

In most modern labs, Type II water is only used for rinsing glassware, and as feed water to a Type I DI polishing system. Type IV reagent grade water (typically generated by reverse osmosis) has become meaningless, in that it has almost no practical use in the modern laboratory other than as feed water to a Type I or Type II DI system.

While “distilled water” is still used in some undergraduate and high school laboratories, the high energy and labor costs of a still, combined with the relatively poor quality of the distilled water, are serving to rapidly phase out distillation as a practical technique for purifying water. “Standard Methods” points out that there are significant problems with distilled water in that the quality is dependent on the incoming tap water quality, and upkeep on the still can be difficult, and distilled water degrades in the storage vessel.

In discussing reagent grade water, Section 9020 of “Standard Methods” advises as follows:

“To avoid contamination, do not store such water.”

“Stills produce water that characteristically deteriorates slowly over time as corrosion, leaching and fouling occur.”

“Stills efficiently remove dissolved substances but not dissolved gases or volatile organic chemicals.”

“Freshly distilled water may contain chlorine and ammonia.”

The National Committee for Clinical Standards (NCCLS) states in Section 7.1 and 7.2 that:

“Type I water should be used immediately after processing.”

“Type I water cannot be stored because its resistivity will decrease, metals and/or organic compounds will be leached from the storage container, and bacterial contamination will occur."

"Storing water in large vessels (carboys) for extended period of time is unacceptable because of the inevitable, unpredictable rate of degradation of the water quality."